1. Field of the Invention
The invention relates to an image pickup apparatus that can be suitably applied to a video camera or the like with an automatic knee processing function, for example, and also relates to an image signal processing apparatus and an image signal processing method used by such apparatuses.
2. Description of the Related Art
CCD (Charge Coupled Device) elements and CMOS (Complementary Metal Oxide Semiconductor) elements are currently in widespread use as image pickup elements in image pickup apparatuses such as digital still cameras and video cameras. However, since the output dynamic range of such image pickup elements is narrower than the input dynamic range, there is the problem that when the inputted amount of light exceeds the output dynamic range, so-called “bright-area-gradation deterioration” occur in images displayed on a display unit or the like.
FIG. 1 is a graph showing the output characteristics of an image signal outputted from an image pickup element. In FIG. 1, the horizontal axis shows the input dynamic range (%) and the vertical axis shows the output dynamic range (%). In FIG. 1, the maximum value in the output dynamic range (hereinafter referred to as the “white clip point”) is shown by the broken line labeled “WP”. In the example shown in FIG. 1, the white clip point WP is 109%, and the input dynamic range and the output dynamic range match each other one to one until 109% is reached. That is, an output signal that is in proportion to the inputted amount of light is outputted.
However, once the inputted amount of light exceeds the white clip point WP, the value of the output dynamic range no longer changes. That is, even if an amount of light that exceeds the white clip point WP is received, only a fixed value will be outputted from the image pickup element. This means that high luminance components where the white clip point WP is exceeded will be deteriorated in bright-area-gradation (so that highlights portions appear pure white) when displayed on a monitor or the like.
In the past, to suppress bright-area-gradation deterioration, processing has been carried out to compress high luminance components of a predetermined level or higher to keep the brightness (i.e., luminance) of the subject within the output dynamic range. Such processing is called “knee processing”. FIG. 2 shows the image signal output characteristics when knee processing has been carried out. In FIG. 2, the point labeled KP shows the start point for the knee processing and is called the “knee point”. In the example shown in FIG. 2, the knee point KP is at the point of 88% of the input dynamic range, and high luminance components in a range of 88% to 160% of the input dynamic range are compressed to within 88% to 109% of the output dynamic range. The output characteristics curve of the image signal is bent on reaching the knee point KP, and the slope (i.e., inclination) of the curve corresponds to the amount of compression in each range of the input dynamic range where knee processing has been carried out. This slope is called the “knee slope”.
FIG. 3 is a histogram showing a distribution of numbers of pixels by luminance level in a state where knee processing has not been carried out, and FIG. 4 is a histogram showing a distribution of numbers of pixels by luminance level in a state where knee processing has been carried out on an image signal composed of the pixels shown in FIG. 3. In these histograms, the vertical axis shows the number of pixels and the horizontal axis shows luminance levels (%). In FIG. 3, since knee processing is not carried out, all of the pixels to the right of the line at the white clip point WP are displayed on a display with bright-area-gradation deteriorated.
On the other hand, FIG. 4 shows that by carrying out processing that compresses the high-luminance components in the range from 88% (which is set as the knee point as shown in FIG. 2) to 160% of the input dynamic range into the range from 88% to 109% of the output dynamic range, the number of pixels with a luminance level of 150% or higher falls, and such pixels with the luminance level of 150% are replaced by pixels with a luminance level of 50 to 100% and pixels with a luminance level of 100 to 150%. By carrying out this processing, it is possible to reduce the total number of pixels with luminance values that are equal to or higher than the white clip point WP, so that it is possible to suppress bright-area-gradation deterioration.
FIG. 5A shows an example of an image where knee processing has not been carried out and FIG. 5B shows an example of an image produced by carrying out knee processing on the image shown in FIG. 5A. Although the part labeled as the region AR1 in FIG. 5A is deteriorated in bright-area-gradation, by carrying out knee processing to compress the high-luminance components, it is possible to correct the image to an image with suppressed bright-area-gradation deterioration, such as that shown in FIG. 5B. Also, image pickup apparatuses with a function for automatically adjusting the knee point in accordance with the luminance level of the subject have been introduced in recent years. Such function is called an “automatic knee” or “autoknee”.
A method that assigns a wider output dynamic range to image levels that appear frequently in a frame is also conceivable. In such method, the inclination of the knee slope is automatically controlled.
Japanese Unexamined Patent Application Publication No. H08-181887 discloses adaptive control over the knee slope from the knee point upward in accordance with the histogram of the input image.